The invention relates to a method for the simultaneous removal of SO.sub.2 and NO.sub.x from exhaust or combustion gases.
Typically, flue gases resulting from the combustion of carbonaeous material in power plants contain hundreds to thousands of parts per million (PPM) of SO.sub.2 and several hundred PPM of NO.sub.x mostly in the form of NO.
In a typical process, Sulfur Dioxide is removed from exhaust gas by wet-scrubbing. In one method, the flue gas is passed through an aqueous slurry of lime or limestone which reacts with the SO.sub.2 to form insoluble CaSO.sub.4 and CaSO.sub.3. In a variation, known as the dual alkaline method, the flue gas is passed through a scrubber solution containing a sulfur dioxide absorbent such as sodium carbonate which reacts with the SO.sub.2 to form NAHSO.sub.3. The scrubber solution is regenerated by contact with a lime or limestone slurry where the NAHSO.sub.3 reacts with the calcium to form large quantities of insoluble hydrated CaSO.sub.3 and CaSO.sub.4. These insoluble salts typically are sent to landfills for disposal and thereby add to the cost of the process. One other disadvantage is that this process is not effective in removing oxides of nitrogen, particularly NO, from exhaust gases.
In one effort to simultaneously remove oxides of sulfur and oxides of nitrogen (SO.sub.x and NO.sub.x) from exhaust gases, the gas was contacted with an aqueous absorbent containing a metal sulfite, ferrous ions and an aminopolycarboxylic acid, usually ethylenediamine tetraacetic acid. The oxides of sulfur are removed as dithionates (M.sub.2 S.sub.2 O.sub.6) in a crystallization separation. However, the dithionates are difficult and expensive to decompose.
In other processes (for instance U.S. Pat. No. 4,082,835) electrodialysis is employed to regenerate the spent liquid absorbent with the liberation of concentrated SO2 gas. A second electrodialysis unit or some other process is required to convert the waste sulfate to a useful by-product or to a form for disposal. Such processes have not involved the removal of both oxides of nitrogen and oxides of sulfur. Furthermore, they have not permitted the regeneration of SO.sub.2 absorption capacity and the conversion of waste sulfate in a single electrodialysis unit.
Therefore, in view of the above discussion of the disadvantages of prior processes, it is an object to the present invention to provide an improved process for the removal of both oxides of sulfur and oxides of nitrogen from exhaust gases.
It is a further object to provide a process for NO.sub.x and SO.sub.2 removal in which the SO.sub.2 is conveniently converted to sulfuric acid in the regeneration of the process absorbent.
It is also an object to provide an SO.sub.x and NO.sub.x removal process in which the disposal or decomposition of insoluble calcium sulfates or alkaline metal dithionates are avoided.
It is yet a further object to provide an improved electrodialysis process for the regeneration of an absorbent liquid for oxide of sulfur gas.
In accordance with the present invention, a method of removing oxides of sulfur and oxides of nitrogen from exhaust gases includes the steps of contacting the exhaust gas with a liquid absorbent including a metal chelate and a solution of sulfite and bisulfite species. The metal chelate is specific for oxides of nitrogen while the sulfite solution absorbs oxides of sulfur. After the liquid absorbent is spent, it is regenerated first by electrodialysis involving passing it through a first compartment defined between a bipolar membrane and an anion selective membrane. At the same time, a solution of sulfuric acid is passed through an adjacent compartment, defined between the anion selective membrane and a second bipolar membrane. By imposing a source of electrical potential across the adjacent compartments, water is split at the bipolar membrane to provide hydroxide ions into the liquid absorbent and hydrogen ions (H.sub.3 O.sup.+) into the sulfuric acid solution. Bisulfite or sulfite anions are transferred from the liquid absorbent into the acid solution at the same time. This reduction in the bisulfite anion concentration regenerates the liquid absorbent capacity in respect to oxide of sulfur gases. The oxides of nitrogen are stripped from the liquid absorbent into a flow of oxygen containing gas to regenerate the capacity of the liquid absorbent for oxides for nitrogen. The regenerated absorbent liquid is recycled into contact with the exhaust gases in the continuous removal of oxide of nitrogen and oxide of sulfur pollutants.
In more specific aspects of the invention, the absorbent liquid includes an aqueous solution of sulfite and bisulfite ions in ratio of about 1 to 10 balanced by cations of sodium, potassium or ammonia.
In further aspects of the invention, the metal chelate is an aminopolycarboxylic acid with ferrous or zinc, preferably zinc, substituted therein. The aminopolycarboxylic nitrilotriacetic acid, diethylentriamine-N,N,N',N",N"-pentaacetic acid, nitrilotripropionic acid, 1,2-propylenediamine-tetra-acetic acid, N-hydroxyethylethylene diamine-N,N',N' triacetic acid, ethylenediamine-tetrapropionic acid, or ethylenediamine tetraacetic acid (EDTA).
In other aspects of the invention, the electrodialysis regeneration of the liquid absorbent in respect to oxide of sulfur capacity is performed in an electrodialysis stack including only bipolar membranes and anion selective membranes. Hydroxide ions thus are added into the liquid absorbent and bisulfite ions are transferred from the liquid absorbent into the sulfuric acid solution.
In further aspects of the invention, sulfuric acid solution containing sulfurous acid is contacted with an oxygen containing gas to oxidize the sulfurous acid to sulfuric acid.
In other aspects, the oxygen containing gas consists essentially of air substantially free of carbon dioxide. The oxidation is performed in the presence of a catalyst such as V.sub.2 O.sub.5, Fe.sub.2 O.sub.3 or CuO.
In futher aspects of the invention, the liquid absorbent with regenerated capacity for sulfur oxides is contacted with a flow of air at in excess of 70.degree. C. while the liquid absorbent is at a pH in excess of five.